Nanomaterials have been widely used in various areas due to the unique properties. Magnetic nanoparticles can be applied in many industries, such as biomedical, magnetic identification and data-storage systems, depending on the size and magnetic properties. More specifically, magnetic nanoparticles can be used as contrast agents in MRI by using the interaction between magnetic field and hydrogen nucleus. Magnetic nanoparticles can also be used as a local heater to kill tumor cells under magnetic field with a certain frequency. Magnetic nanoparticles with long relaxation times can be used as information recorder in data-storage field.
More specifically, magnetic nanoparticles with controlled size and high magnetization, are demonstrated as important materials with wide applications in magnetic recording, magnetic resonance imaging (MRI), drug delivery and therapy. However, there are many problems of the application of bare iron oxide nanoparticles, such as easy aggregation, quick biology-caused decomposition and the further loss of the magnetic property. Silica coated iron oxide core-shell structure magnetic nanoparticles provide the protecting biocompatible silica shell, which is also a platform for subsequent surface functionalization via a powerful silica surface chemistry toolbox.
There are several traditional methods to prepare the iron oxide/silica shell magnetic nanoparticles, namely coprecipitation of Fe(II), Fe(III) salts and tetraethyl orthosilicate (TEOS) in microemulsion or thermal decomposition of Fe(III) chelate under high temperature resulting in iron oxide nanoparticle seed followed by hydrolysis of TEOS on the particle surfaces. The first strategy usually generates magnetic nanoparticles with poor magnetization which cannot meet the needs for real applications. The second method requires harsh condition: high temperature (at least above 200° C., usually around 265° C.), which is an obstacle for its scale-up fabrication in industry. It is also known that the magnetization values are critically dependent on the size of the nanoparticle with lower values being measured at smaller sizes. Thus, it is a challenge to prepare nanoparticles with small sizes and high magnetization values.
As such, a need exists for a method of forming magnetic nanoparticles under mild conditions suitable for scale-up fabrications in industrial uses, while still resulting in high saturation magnetization.